The present invention relates to optical switch arrays and, more particularly, to an optical switch array, of particularly compact geometry, in which arbitrary combinations of the inputs and outputs are explicitly addressable.
Integrated optical switches are well-known. For an early review of the art, see Lars Thylen, xe2x80x9cIntegrated optics in LiNbO3: recent developments in devices for telecommunicationsxe2x80x9d, Journal of Lightwave Technology vol. 6 no. 6 (June 1988), pp. 847-861. Waveguides are created in a lithium niobate substrate by processing the substrate locally to increase the index of refraction. For example, the index of refraction of lithium niobate may be increased locally by diffusing titanium into the substrate. To divert light from one waveguide to another, the waveguides are coupled by local optoelectrical manipulation of their indices of refraction. Well-known examples of optoelectrical switches include directional couplers, BOA couplers, digital-optical-switches and x-switches. Depending on the voltage applied to such a switch, light is thus partly or completely diverted from an input waveguide to an output waveguide.
By appropriately combining waveguides and switches, a switch array is formed to switch light from a plurality of input waveguides among a plurality of output waveguides. A variety of switch array geometries are known. FIG. 1A is a conceptual illustration of a switch of one such geometry: crossbar geometry. A set of input waveguides 10 crosses a set of output waveguides 12. At the crossing points, the waveguides are coupled by 2xc3x972 switches 14. For simplicity, only four input 5 waveguides 10 and four output waveguides 12 are shown in FIG. 1A. Typically the numbers of input waveguides 10 and output waveguides 12 are equal powers of 2, up to a practical maximum of 32.
FIG. 1B shows, schematically, the actual layout of the switch array of FIG. 1A. Switches 14 are shown as directional couplers, in which parallel segments of the waveguides are flanked by electrodes (not shown) to which the coupling voltages are applied. Note that input waveguide 10a leads directly into output waveguide 12a, that input waveguide 10b leads directly into output waveguide 12b, that input waveguide 10c leads directly into output waveguide 12c, and that input waveguide 10d leads directly into output waveguide 12d. To allow arbitrary coupling of inputs to outputs, three auxiliary waveguides 11a, 11b and 11c are provided. Waveguides 10a-12a and 10b-12b are coupled in switch 14a. Waveguides 10b-12b and 10c-12c are coupled in switches 14b and 14c. Waveguides 10c-12c and 10d-12d are coupled in switches 14d, 14e and 14f. Waveguides 10d-12d and 11a are coupled in switches 14g, 14h, 14i and 14j. Waveguides 11a and 11b are coupled in switches 14k, 14l and 14m. Waveguides 11b and 11c are coupled in switches 14n and 14o. Note that switches 14g, 14k and 14n actually are 1xc3x972 switches, that switches 14j, 14m and 14o actually are 2xc3x971 switches, and that there is no switch corresponding to the lowermost 2xc3x972 switch 14 of FIG. 1A. (A 1xc3x972 switch is a 2xc3x972 switch with one input deactivated; a 2xc3x971 switch is a 2xc3x972 switch with one output deactivated.)
Switch arrays based on geometries such as the crossbar geometry of FIGS. 1A and 1B can be used to divert input signals to output channels arbitrarily. Signals from any input channels can be directed to any output channel, and even to multiple output channels, in broadcast and multicast transmission modes.
Despite the conceptual simplicity of the crossbar geometry of FIGS. 1A and 1B, this geometry has been found inferior, in practice, to two other geometries, the tree geometry, illustrated in FIG. 2, and the double crossbar geometry, illustrated in FIG. 3. FIG. 2 shows the tree geometry, for four input waveguides 20 and four output waveguides 22. Waveguides 20 lead into a binary tree of 1xc3x972 switches 24. Waveguides 22 emerge from a complementary binary tree of 2xc3x971 switches 26. The highest order branches of the binary trees are connected by intermediate waveguides 28. FIG. 3 shows the double crossbar geometry, for four input waveguides 30 and four output waveguides 32. Each input waveguide 30 traverses four 1xc3x972 switches 34a, 34b, 34c and 34d. Each output waveguide 32 traverses four 2xc3x971 switches 36a, 36b, 36c and 36d. The remaining outputs of switches 34 are connected to respective inputs of switches 36 by intermediate waveguides 38. Note that, in principle, switches 34d and 36a are not needed, because input waveguides 30 could lead directly to switches 36d and output waveguides 32 could emerge directly from switches 36a; but, in practice, the illustrated configuration has been found to reduce cross-talk.
The tree and double crossbar geometries require larger numbers of switches than the equivalent crossbar geometry. Nevertheless, the tree and double crossbar geometries have certain advantages over the crossbar geometry:
1. The tree and double crossbar geometries have lower worst-case crosstalk than the crossbar geometry.
2. In general, the path from a particular input waveguide to a particular output waveguide through a crossbar switch array is not unique. Therefore, computational resources must be devoted to reconfiguring a crossbar switch array in real time. In a tree switch array or in a double crossbar switch array, the path from any particular input waveguide to any particular output waveguide is unique, so it is trivial to compute how to reconfigure such a switch array in real time.
3. To prevent loss of optical power by radiation, the intermediate waveguides of an optical switch array must have gentle curvature. In the case of the crossbar geometry, this requires that the switches be arranged in a diamond pattern, as illustrated in FIGS. 1A and 1B. This is a less efficient packing of the switches than, for example, the rectangular matrix pattern of the double crossbar switch as illustrated in FIG. 3.
According to the present invention there is provided an optical switch array including: (a) at least three output waveguides; (b) a first group of at least three input waveguides; (c) for each of the input waveguides of the first group: for each of the output waveguides, a combining element coupling the each output waveguide only to the each input waveguide; and (d) for each of the input waveguides of the first group, a switching mechanism for coupling all of the output waveguides to the each input waveguide; the output waveguides, the input waveguides, the combining elements and the switching mechanism all being arranged substantially in a common plane; all of the output waveguides traversing successively respective the combining elements in a common order relative to the input waveguides of the first group.
According to the present invention there is provided a method for switching signals to at least one of at least three output channels from at least one of at least three input channels, each input channel providing signals to only one output channel, including the steps of: (a) providing an optical switch array including: (i) at least three output waveguides, each of the output waveguides corresponding uniquely to one of the output channels, (ii) at least three input waveguides, each of the input waveguides corresponding uniquely to one of the input channels, (iii) for each of the input waveguides: for each of the output waveguides, a combining element coupling the each output waveguide only to the each input waveguide, and (iv) for each of the input waveguides, a switching mechanism for coupling all of the output waveguides to the each input waveguide, the output waveguides, the input waveguides, the combining elements and the switching mechanism all being arranged substantially in a common plane, all of the output waveguides traversing successively respective the combining elements in a common order relative to the input waveguides; and (b) for each of the input waveguides corresponding to an input channel wherefrom a signal is to be switched: setting the switching mechanism to divert at least a portion of the signal to the output waveguide corresponding to the output channel whereto the signal is to be switched.
According to the present invention there is provided a method for multicasting from at least one of at least three input channel to at least two of at least three output channels, each output channel receiving input from only one input channel, including the steps of: (a) providing an optical switch array including: (i) at least three input waveguides, each of the input waveguides corresponding uniquely to one of the input channels, (ii) at least three output waveguides, each of the output waveguides corresponding uniquely to one of the output channels, (iii) for each of the input waveguides: for each of the output waveguides, a combining element coupling the each output waveguide only to the each input waveguide, and (iv) for each of the input waveguides, a switching mechanism for coupling all of the output waveguides to each input waveguide, thereby coupling the input channel corresponding to each input waveguide to the output channels, the input waveguides, the output waveguides, the combining elements and the switching mechanisms all being arranged substantially in a common plane, all of the output waveguides traversing successively respective combining elements in a common order relative to the input waveguides; and (b) for each input channel wherefrom a signal is to be multicast: for each of the at least two output channels whereto the signal is to be sent, setting the switching mechanism, that couples the each input channel to the each output channel, to divert only a portion of the signal to the each output channel.
We have discovered that, by rearranging the connections of the double crossbar geometry of FIG. 3, a new geometry is obtained that allows a spatially more compact configuration of switches and interconnecting waveguides. Compactness is an important consideration, because it allows a larger switch array (more inputs and outputs) to be fabricated on a substrate of a given size. One substrate suffices for a switch array of the present invention that is functionally equivalent to a prior art switch array that may require two (double crossbar geometry) or three (tree geometry) substrates.
FIG. 4 shows the geometry of a switch array of the present invention, in the case of four input waveguides 40 and four output waveguides 42. As in the double crossbar geometry of FIG. 3, each input waveguide 40 traverses four 1xc3x972 switches 44, each output waveguide 42 traverses four 2xc3x971 switches 46, and the remaining outputs of switches 44 are connected to respective inputs of switches 46 by intermediate waveguides 48. Unlike the double crossbar geometry of FIG. 3, switches 46a all are traversed by the same output waveguide 42a, switches 46b all are traversed by the same output waveguide 42b, switches 46c all are traversed by the same output waveguide 42c, and switches 46d all are traversed by the same output waveguide 42d, so that all input waveguides 40 are coupled to output waveguides 42 in the same order: first to output waveguide 42a, then to output waveguide 42b, then to output waveguide 42c, and finally to output waveguide 42d. This allows intermediate waveguides 48 that lead to a particular output waveguide 42 to be geometrically adjacent, with a corresponding increase in the compactness of a switch array of the present invention as compared to an equivalent double crossbar switch array.
As in the double crossbar geometry of FIG. 3, strictly speaking, 1xc3x972 switches 44d and the first 2xc3x971 switches 46 traversed by output waveguides 42 are not necessary, and are present only to reduce cross-talk. Co-pending U.S. patent application Ser. No. 09/085,369 teaches a similar switch array geometry, in which these switches are in fact not present.
In the days before integrated optics, Fulenwider, in U.S. Pat. No. 3,871,743, described an optical switch array having a topology similar to that of the present invention. Unlike the present invention, the particular embodiment described by Fulenwider is not well-suited to fabrication as an integrated optical device. By contrast, a switch array of the present invention is easily fabricated, essentially in a single plane, as an integrated optical device, for example on a Z-cut lithium niobate substrate.
1xc3x972 switches 44 and 2xc3x971 switches 46 are indicated on FIG. 4 for illustrative purposes only. More generally, the scope of the present invention includes any suitable switching element in the role of 1xc3x972 switch 44 and any suitable coupling element in the role of 2xc3x971 switch 46. In particular, passive y-junction combiners may be substituted for 2xc3x971 switches 46.
It will be appreciated that the switch array of FIG. 4 is reversible, in the sense that the roles of input and output can be exchanged. FIG. 4 still serves to illustrate such a reversed switch array, with reference numeral 40 designating output waveguides, reference numeral 42 designating input waveguides, reference numeral 44 designating 2xc3x971 switches, and reference numeral 46 designating 1xc3x972 switches. The scope of the present invention includes any suitable combining element in the role of 2xc3x971 switch 44 and any suitable switching element in the role of 1xc3x972 switch 46. In particular, passive y-junction combiners may be substituted for 2xc3x971 switches 44.
To switch signals from an input channel, associated uniquely with a corresponding input waveguide, to one or more output channels, each output channel being associated uniquely with a corresponding output waveguide, the output waveguides are considered in turn. For each output waveguide, the switching element that couples the input waveguide associated with the desired input channel is set to divert the appropriate portion of the input signals of that channel to the target output waveguide. If signals from other input channels are to be switched to other output waveguides, then the corresponding other switching elements associated with the target output waveguide are set to pass those signals without diversion.
Switching signals using the reversed switch array is even simpler. To switch signals from an input channel, associated uniquely with a corresponding input waveguide, to one or more output channels, each output channel being associated uniquely with a corresponding output waveguide, the input waveguides are considered in turn. For each input waveguide, the switching elements that couple that input waveguide to the desired output waveguides are set to divert the appropriate portion of the input signal from that input waveguides channel to each of the desired output waveguides, and all the other switching elements of that input waveguide are set to a pass-through state.